The present invention relates to the art of electrical filter arrangements, and more particularly to a digitally tuned filter circuit including a plurality of capacitors which may be selectably coupled to one of several tuned filters so as to incrementally vary the tuning frequency of the filter.
Radio receivers commonly include circuits, known as preselector circuits, which are interconnected at a point in the signal path prior to the receiver. These preselector circuits are tunable and are included for the purpose of providing an initial stage of frequency selection so as to eliminate out-of-band frequency components. Similar circuits, known as post-selector circuits, are commonly included in transmitters between the exciter stage and the power amplifier for similar reasons. Currently, the control of these pre- and post-selector circuits is accomplished by mechanical servo drive systems. These systems not only tend to be quite expensive but also operate very slowly, particularly for widely-spaced frequencies.
Efforts have therefore been directed to providing a non-mechanically tuned circuit meeting speed and flexibility requirements. The prior art in this area is exemplified by the following U.S. patents: Jensen et al., U.S. Pat. No. 3,166,726; Nilssen et al., U.S. Pat. No. 3,518,586; Keese, U.S. Pat. No. 3,651,469; Imazeki, U.S. Pat. No. 3,794,925; Imazehi et al., U.S. Pat. No. 3,895,303; Van Anrooy, U.S. Pat. No. 3,913,020; and Pflasterer, U.S. Pat. No. 3,961,261.